Implicit Safety Factors in Wood Design 3

[Hickory]

It looks to me like the baseline design values for wood compression perpendicular to the grain are way too high. For white oak dimension lumber, for example, the design value is 800 psi (American Wood Council: Design Values for Joists and Rafters, 2005 edition, pg. 18), whereas the elastic limit found by testing small samples of clear wood is 1073 psi (Wood Handbook, pg. 4-11). This means that an implicit safety factor of SF = 1073/800 = 1.34 is hidden in the design values table. Back in the old days, however, the safety factors applied in such situations were never less than 4, and the use of 5, 6, or more was not unusual. (See

http://chestofbooks.com/architecture/Building-Trades-Pocketbook/Wooden-Columns.html.)

If we are going to take decisions about safety factors out of the hands of the engineer on the scene who is familiar with the details of the situation and place them in the hands of people in a smoke filled room thousands of miles away, shouldn't those people at least acknowledge their lack of on-the-scene information by using implicit safety factors that fall at the high end of the traditional range, rather than choosing values that fall far below the bottom of that range?

Can anybody provide a rationale for what is going on here? Is safety no longer a concern in what passes for structural engineering nowadays?

 

[frv]

The tabulated stress is at the proportional limit. This is NOT what is used for design for compression perpendicular to the grain, as "failure" is more of a crushing failure. As stated in the wood handbook, "there is no clearly defined ultimate stress for this property". If I remember correctly, I think Breyer discusses what is considered "failure".

On a separate note, it really gets old to hear a lot of the more senior engineers complain about the "state" of engineering nowadays. Really? I bet I learned a lot of things you didn't in school. That's not to say I'm a better engineer or that I'm in any way a practical engineer (yet). But engineering is always changing and will always change with technology and new knowledge. Instead of complaining, you should try to mentor a younger engineer and teach him why engineering is such B.S. nowadays.

I bet you also complain about the devaluation of an A and how music sucks nowadays.

 

[woodman88]

"Back in the old days, however, the safety factors applied in such situations were never less than 4, and the use of 5, 6, or more was not unusual."
Back in the old days (pre 1991) they got the best piece of lumber they could find to test and divided the values by a large factor to get what “somebodies” thought would be a good design value to use. Today they test random samples of the “as sold” lumber to test what design values should be used.
I prefer how it is being done today.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[abusementpark]

Wood bearing failures aren't usually a matter of life safety.

 

[Cadair]

I agree with the above posters. Wood bearing usually isn't a matter of life safety, so they are average values given per ASTM D 143 which has a 0.04 in. deformation limit.

Hickory, are you suggesting that each project engineer makes their own design values for each project? (If you think this is easy, please give me the bending strength of a Southern Yellow Pine, No. 2, 2x4.) Instead of reinventing the wheel each time, isn't it easier to figure out what the people in the "smoke filled room" did and see if it applies? (Is it a life safety situation or isn't it?) And adjust from there?

 

[LonnieP]

If I remembeer correctly, there are NO increases allowed, of any kind, for the currently tabulated bearing values perpendicular to grain. Back in the "good ole days" you got 385 psi, plus short term increases, for Douglas Fir. Now it's 620 psi, period. I see engineering advantages both ways.

LonnieP

 

[MiketheEngineer]

Back in the good old days using bell curves and 5% exclusion factor - the FS was probably in the 1.67 to 2.0 range.

Things have changed and hopefully more reliable than before.

 

[Hickory]

To LonnieP:

I'm not saying you should feel free to increase the design values that apply in your locality. Such values are inputs to the calculations by which you determine the load-bearing capacity a structural element is required to have in your situation. This requirement is a matter of law. You may think a particular structural member can support a larger load than the law allows, but if you act on that opinion, you can get yourself into trouble.

What I am saying is that "what the law allows"--i.e., the capacity (a.k.a., "adjusted allowable stress") that you come up with for a structural member may be too high for the application you have in mind, rather than too low. The reason lies in the variability of the structural materials that are available, most particularly including wood. Wood is produced in nature. It is not a manufactured item for which quality is subject to control at every stage of production. As a result, there is always a distinction between the load-bearing capacity that you want a structural member to have, based on calculations you made while sitting at your desk (or that your computer made), and the capacity which the actual, real world piece of wood which you ultimately install is going to have. The probability is always going to be greater than zero that the installed member will not be as strong as you wanted, and will fail. Result: you may have to "overengineer" to take the likely consequences of that failure into account.

The traditional way to do that is to employ safety factors. If failure means a crack in a picture window, then it's no big deal, and you can go with the NDS specs, subject to whatever changes may apply due to local building codes. But if the failure means a hundred people will die, then the implicit safety factor hidden in the design values table may not be large enough. You may need to go with something larger.

There I go using the word "larger" again. That apparently caused confusion before, so let me emphasize that a safety factor is a divisor, not a multiplier. To revisit the example I mentioned in my original post, the white oak design value for compression perpendicular to the grain is 800 psi, while the corresponding measured strength of small clear samples is 1073 psi. That means the measured strength has, in effect, been divided by a safety factor, SF, to produce the design value, as follows:

1073/SF = 800

Thus SF = 1073/800 = 1.34

The measured value is divided, not multiplied, by the safety factor, to produce the design value. (If you want to multiply, you can use a strength ratio, SR, such that SR = 1/SF, but that's not what I am talking about.)

Anyway, the bottom line on all of this, and the main thing I was getting at in my original post, is that you can't just blindly trust the design values given in the NDS specs. No, you can't increase them. That is illegal. But you can, and must, be alert to cases where the design values are too high and need to be reduced. In such cases the implicit safety factors hidden in the tables are either too low in general (as in the case for compression perpendicular to the grain, where a safety factor of 1.34 is simply ridiculous) or else too low for your specific application.

In my view, the current practice of hiding safety factors in design value tables is totally wrong. It discourages structural engineers from thinking about safety, hence from developing judgment about safety, and encourages blind trust in the one-size-fits-all decisions made in smoke-filled rooms by people who have no information about the safety related considerations that apply to a specific structural element at a specific job site.

I realize that there was no way to stop this trend. Among the big builders there has always been a morally challenged group who wanted to save money by throwing up nice-looking crap and selling it to members of the general public, members who are ignorant and trusting by nature. Thus they pushed for government intrusion into the building trades. They wanted decisions about safety handed over to committees in smoke-filled rooms, whose members were chosen on the basis of their corruptibility--i.e., their susceptibility to bribery and/or blackmail--and they have accumulated immense fortunes by their efforts. It has become, in effect, legal to sell unsafe crap to members of the general public, and engineers of the old school, who were experts on safety because they thought about it every day, are for the most part gone, replaced by a new generation of people who have been taught to blindly trust decisions made by "authorities" far removed from the job site.

Nevertheless, susceptibility to such a trend has not been in the engineer's best interests. It will be the engineer of record who is left holding the bag, when the consequences of this madness begin to manifest themselves, as they are already doing. It is always the little guy who takes the fall, and, for the naive wet-behind-the-ears "structural engineers," eager to "succeed" by fitting in, who don't know who that is, here's a heads up: the fall guy is you.

 

[woodman88]

Hickory, From the Arizona State Board of Technical Registration “A registrant shall apply the technical knowledge and skill that would be applied by other qualified registrants who practice the same profession in the same area and at the same time.” Please check your state rules and statutes for similar requirements of practice.
Based upon the above Board statement, what are your qualifications concerning lumber?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[frv]

Wow, Hickory, you need a tissue for your issue.

Smoke-filled rooms? Curruptibility? Have you ever attended a meeting? Have you ever met a member? I've been fortunate to know several committee members. To a person they have been honest, hard-working people whose sole goal is to improve the profession. Perhaps the fact that these members (at least those in academia) are much smarter than you gives you an inferiority complex. I don't know, but it is patently unfair to describe them as you have.

I suggest you do some additional reading. Most of what is decided in these "smoke-filled" rooms is done so based on research you can access. Have you heard of the concept of a "reliability index" with respect to structural failures? Are you aware of statistical distribution of the design values for wood members? Of the statistical distribution of the actual live loads? How the intersection of these values is used for the basis of design? If anything, my generation is more informed of the process and the basis of design than previous ones. We are trained to understand the fundamental issues that are addressed in, and, more importantly, the limitations of the codes.

The fact that you keep insisting that the proportional limit (not even the elastic limit) somehow relates in any way to a safety limit for bearing perpendicular to the grain leads me to believe you don't understand this failure mode particularly well. Well, the same people who explained to me why the elastic limit is meaningless in this particular limit state are the same ones who volunteer their time to make life easier for the rest of us. I think gratitude is more appropriate than cynicism.

 

[Hickory]

To FV:

This in reference to issues raised in your first post.

In the case of materials (e.g., wood) for which the stress-strain curves have a straight-line portion, the proportional limit (the largest stress on the straight line) and the elastic limit (the stress beyond which the object will not return to its original shape) are approximately equal. Because the elastic limit is only slightly larger than the proportional limit yet much harder to measure, the proportional limit serves as a conservative measure of the elastic limit. From the conceptual standpoint, however, the focus should be on the elastic limit, for the reasons that I will explain below.

To begin let me address the common claim, referenced by you, that in the case of compression perpendicular to the grain no clearly defined failure mode exists.

Despite the fact that it is a principle mostly honored in the breach in the "modern" era, I believe that structures ought to retain their design properties throughout their intended lifetimes. This principle leads directly to the rule that no structural member should be used in a manner that would result in its elastic limit being exceeded. The reason: as soon as it passes its elastic limit, it suffers irreversible damage, after which it no longer retains the properties assumed in the designer's calculations. For those who accept this philosophy of design, failure of a structural member occurs by definition as soon as its elastic limit is exceeded, whether there are significant adverse consequences associated with that failure or not.

The justification for such a definition of failure arises out of considerations similar to the rule that you should never point a gun, loaded or unloaded, at anyone you do not intend to kill. The rule is justified because if it is followed, accidental deaths by gunshot become virtually impossible. And by the same token, the rule that no structural member should be used in a manner that causes its elastic limit to be exceeded is justified because if it is followed, structural failures become virtually impossible, including those which are associated with significant adverse consequences.

Properly defined, in short, failure occurs when a structural member exceeds its elastic limit. Acknowledged or not, this is a fundamental safety-related consideration in the field of structural design. Thus it is not merely wrong to say that no clearly defined failure mode exists for compression perpendicular to the grain, it is also dangerous, because such a denial reveals exactly the same thing about the speaker that would be revealed if he said: "It is OK to point a gun at a person you do not intend to kill, if the gun is unloaded." To be explicit: it reveals that the speaker lacks a clear awareness that what he believes, and reality, are fundamentally not the same thing, that the best he can hope for is to achieve correspondence between them, that a careful and sustained process of reasoning is necessary to accomplish that end, and that failure to do so may have serious consequences.

The key point is that design values are just numbers that a designer uses as inputs to his calculations. Once the calculations are completed and the final, fully-adjusted design values (allowable stresses) are known, actual structural members must be obtained which equal or exceed those calculated values. This is where the difference between what the designer thinks and reality enters in: whatever his calculations, the probability is always going to be greater than zero that the intended load will exceed the strength of the actual structural member that is installed. That's where the safety factor comes in. The larger the safety factor that you use, the smaller the calculated capacity of any specific structural member is going to be. Since the intended load does not change, the use of the safety factor narrows your choices. Structural members that would have been deemed able to carry the load, prior to dividing their capacities by the safety factor, will be deemed unsuitable after doing so. Thus the use of the safety factor forces you to choose larger, stronger structural members than you would have chosen otherwise. Result: it becomes more likely that, even if the structural member chosen is defective, it will still be able to support the intended load.

You "overengineer," in short, because you are very aware of, and willing to acknowledge, your own fallibility.

As I have pointed out previously, our rulers have tried to take the choice of safety factors out of the designer's hands. They have done that by putting forth tables of "design values" that contain implicit, hence hidden, safety factors. And their system is a "one size fits all" kind of thing: for those who operate within it, the probability of installing a structural element that will fail is going to be the same regardless of the safety-related consequences of the failure. It will not matter in the slightest whether such a failure will merely produce a crack in a picture window or will trigger a collapse that kills hundreds of people, because the safety factor is not chosen by the person who is aware of the potential consequences (i.e., by the designer), but by a committee that may be thousands of miles away from the scene.

If you stay within the confines of such a system you can hope that any failure that may occur will not have horrendous consequences, or that if it does you can prevail in the resulting lawsuits by pointing out that what you did was "standard practice," "up to code," etc., and hence was not negligent. And you may even win in court, if you do that. But such a win will not pass the sniff test, and neither will your reputation when the affair is over. And the odds are that your legal costs, which you will be unlikely to recover, will be very large.

So why play that game? The implicit safety factors that our rulers have hidden in their design-value tables are easy to unmask: in the case of wood, you simply divide the design values into the corresponding measured values for clear wood samples. The safety factor is the result. Then, if the safety related risks are trivial, you can always go with the implicit safety factors hidden in the standard table, and use the standard design values as the starting points of your calculations; but if the risks are significant, a more conservative approach may be indicated. In that case you should choose a larger safety factor than the one hidden in the table, so that you can reduce the probability of failure closer to zero.

Anyway, enough of that. Concerning your guess about my attitude toward modern music, I would only note that one of my favorite movie scenes is the one where a black gangster has Bruce Willis tied to a chair and is threatening to torture him, and Willis says: "What are you gonna do, force me to listen to rap music?"

That's funny, right?

 

[frv]

Hickory,

The first rule of holes: when you find yourself in one, stop digging.

I suggest you google "straw man". Learn from it.

Just because you repeat what you have already said, only with incredibly unnecessary verbosity, doesn't give you any more credibility. You claim, in a roundabout way, that your conclusions are self-evident. I suggest you also google "false dichotomy". This is apparently another one of your argumentative techniques.

I guess, then, that you are not aware that steel beam design has for a long time been based on the plastification of the section (albeit in a roundabout way). You design concrete to remain linearly elastic?

I don't claim that there is no clearly defined ultimate stress; the wood handbook does, as I properly cited in my first post. You claim that we should design only in the linearly elastic region, right? You are aware, then, that because it is a natural product (and orthotropic, no less) every single piece of lumber has a different modulus of elasticity. in each direction. along different parts of its length. How do you propose we use lumber given this limitation and your constraint that they should be used solely in the linear region?

Did you ever bother to figure out how the NDS defines failure? Or why?

On a final note, I'm curious to find out what application could possibly have wood perpendicular to grain bearing as a life-safety failure mode. Not saying that there are none, just can't think of any, given wood's natural ductility.

 

[Hickory]

To FRV:

This is in response to your more recent posts.

Concerning my "verbosity," the rule when trying to lead someone to a truth that lies off the beaten path is this:

"First you tell 'em what you're gonna tell 'em. Then you tell 'em. Then you tell 'em what you done told 'em."

Most of the reasons for that procedure are pretty obvious, and apply to everyone: new material can be confusing; repetition aids memory; time is required for thought; etc.

In our case, however, there is an additional problem: I'm old, and you are young. In fact, unless I miss my guess, I've been retired longer than you have been alive.

That is a problem because young adults have just recently escaped from parental control. For close to 20 years, they were subject to being told what was true and false, criticized when their behavior or their opinions did not conform to expectations, and, whether they admitted it to themselves or not, they resented the hell out of the adults who so cavalierly ordered them around.

The roots of this problem trace back to the fact that the vast majority of people enjoy having the power to order other people around, and, in particular, they trace back to the fact that vast numbers of parents are obsessed with forcing their opinions on their children. Since we have a government that has the power to make law, parents have exerted continuous political pressure since the founding of the U.S. in favor of more and more parental control. Result: with the passage of time the age at which a person was legally declared to be an adult--the age at which he became exempt from parental control--has risen. Thus the further back you go in time, the lower the age at which a person reached adulthood. While the age of legal adulthood has varied from state to state, one does not have to go back very far to find states where a person could leave home, get a job, get married, sign binding contracts, etc., at puberty. My paternal grandfather, for example, left home in 1886 at the age of 13 because he couldn't get along with his stepmother. He got a job as a physician's assistant, learned the medical profession via on-the-job training, and went into private practice when he was 20. No college education was required at that time, and he had none, but he became an exceptional physician nonetheless.

The point is this: today's young adults carry a lot more resentment towards their elders than prior generations did, because they have more reason for resentment. They have been ordered around longer, instructed longer, criticized longer, and pressured to conform longer, so they carry far more hostility towards their elders than previous generations did. Result: they are inclined to embrace movements and ideologies that differ from what was done in the past, precisely because they are different. This is, of course, wrong: decisions about what to reject or believe should be based on reason, not resentment.

Nevertheless, I understand those attitudes and I sympathize. Furthermore, I'm not in a position to force you to accept anything I say, and wouldn't do it even if I could. I'm not authoritarian or controlling in the slightest. You won't hear me saying "No one knows wood as good," or anything like that. Instead, I'll give you my reasons when you challenge something I say, and it won't bother me in the slightest if you are not persuaded by my arguments. This is, after all, a discussion group. When disagreements arise, reasons are the only thing that matters. Neither age, experience, IQ, school of matriculation, number of degrees, certification, number of books or papers authored, nor any other characteristic matters. In the final analysis, when the chips are down, the only qualification anyone has for holding an opinion are the reasons upon which he bases that opinion, and I would be the last person in the world to deny that.

So relax. I'm not trying to pressure you into anything. You are free to take whatever you wish from my musings, or to take nothing at all, if that is your inclination.

Concerning my "cynicism" about how the members of regulatory bodies are chosen and what their actual purposes are (as opposed to their stated purposes), I would note that in a free market economy, property rights would be sacrosanct. The building trades would be totally unregulated--laissez faire, as the French used to say. Under those circumstances builders who cut corners to save money and threw up nice looking crap that fell apart in a few years would not have the defense that their structures were "up to code." Moreover, the builder would not have the option of scapegoating the engineer of record in cases where the plaintiff's lawyer argued that special circumstances applied, that in those circumstances the structure should have been stronger than the code required, and that the engineer, unless he was incompetent, "knew or should have known" that to be the case. (If there is no "code" that structures need to be "up to," then it is impossible to turn structural engineers into mindless robots whose sole focus is on ensuring that their designs are "up to code.") Instead, under laissez faire, it would be argued in court that standard conditions are assumed, that there is an implicit warranty of serviceability of the structure to its intended purpose unless the builder explicitly informs the buyer to the contrary. Result: the first such failure would result in the builder being liable for damages, which would be paid by his insurance company. Afterwards, with such a failure on the books, (a) the builder's insurance rates would become prohibitive, forcing him to stand behind his work himself, and (b) subsequent failures on his part related to cutting corners on his designs while still not informing his customers that he was explicitly disavowing any warranty of serviceability--that the structure was being sold "as is"--would result in findings of fraud, and he would face criminal prosecution.

What this means is that fraud would not be a viable business strategy in a laissez faire system.

What "regulatory bodies" do is clear a path through the economic system for sociopathic "businessmen" who intend to "succeed" by means of fraud.

Since that is the effect of building codes, is it not reasonable to consider the possibility that it is also their purpose? Should we not ask ourselves whether it is possible that wealthy people who intend to become even more wealthy by means of fraud might influence the political process in ways that would bring about the creation of regulatory bodies? Could they, perhaps, fund the campaigns of politicians whom they know will comply with their wishes? Could they, perhaps, fund the research of professors who argue that regulation is needed? Could they, perhaps, advertise only in newspapers that support their goals? Could they, perhaps, buy newspapers and employ "journalists" who are inclined to support their goals or whom they can bend to their purposes? And, once regulatory bodies exist, could they not influence how the members are chosen? Could they not make potential members whom they cannot influence appear to be "controversial" by unleashing their kept "experts" and journalistic shills upon them?

And if regulatory committees could be created and controlled by such methods, are there not people who would be willing to employ those methods? Is it not possible that there are sociopaths among the wealthy--people who seek to amass more and more wealth, by any means possible, and who are utterly indifferent to the destruction they might cause?

I hope that you are not so young and naive that you cannot see what I am talking about here, because the people I am describing are in the process of destroying our present global civilization as we speak, and are ushering in a new dark age in which you, but not I, will have to live.

Why am I exempt? Because, as I said earlier, I am old. I could die tomorrow, or next week, or next year. I cannot know the day or the hour, but I can know that the day is near because mortality statistics say that it is. Furthermore, that is in no sense a bad thing. I would not want to live in the future that you face and are denying--the future caused by the dark and sinister men who stand behind those whom you describe as "honest, hard-working people whose sole goal is to improve the profession," and "who volunteer their time to make life easier for the rest of us."

Now let me turn to your belief, which you have expressed in various ways, that structural engineering as presently practiced represents a more highly evolved, improved state, when compared to how things were done in the past.

When I read such comments, what immediately comes to mind is lengthy news footage that I watched back in August of 1992, just after Hurricane Andrew moved across south Florida. It was the immediate aftermath of a major disaster, and all the networks were focused on it. The particular footage that stuck in my mind was taken by a helicopter flying low over residential areas, following along the track of the storm. What it showed was very close to total devastation: thousands upon thousands of wooden homes reduced to their component parts. Lumber, wallboard, shingles, shards of broken glass, furniture, automobiles--all the trappings of suburban life--were strewn in a continuous debris field as far as the eye could see, forming a vast ocean of destruction. The helicopter just flew on and on, for perhaps half an hour or more, and the nature of the scene did not change. In particular, and the most unforgettable aspect of it all--the pattern that stunned me and will stick in my mind as long as I live--was simply this: there was a sprinkling of residential wooden homes that were not destroyed. They just stood there like rocks in the sea of destruction, unaffected by the winds of a category 5 hurricane, while all the homes for miles around them had been swept away.

And here's the kicker: every single such home that I saw in live footage that lasted approximately half an hour, were built in the architectural style common roughly a hundred years ago, back when America was still a free country, when restrictions on builders were mostly unheard of, and when such minor restrictions as existed in a few isolated areas were trivial by comparison to the labyrinthine restrictions prevalent today.

That was my "heads up," the tap on the shoulder that came down with such force that it could not be ignored, telling me that regulation of the building trades, which has been sold to the public based on the pretext that it would make them safer and improve the quality of construction, has in fact been a palpable fraud which has accomplished exactly the opposite of what was promised.

In a free market economy, standard conditions are assumed. If you build a house for a customer, there is an implied warranty of serviceability unless you expressly indicate otherwise. It is expected that you have produced a product which will perform its intended function under the conditions in which it is intended to be used, provided only that it is properly maintained. If you do not indicate otherwise, that is the implicit contract which you enter into with your customer. Standard conditions in Florida, in a free society, would require that a new home be able to withstand a category 5 hurricane, because there is a very real prospect that it will experience one during its intended lifetime. If it won't do that, the builder needs to say so, and say it in an open, explicit, and unmistakeable way. You can't hide it in the fine print. You can't say it in a whisper while the customer is distracted. You need to be open and above board in all respects. Otherwise, when the Big Bad Wolf comes and blows the house down, you are going to be liable. You can't say the home you built was "up to code," because in a free society there ain't no steenking code. You can't call a building inspector to testify at your trial because in a free society there ain't no steenking building inspectors. In a free society, you have only one defense if the house you built falls down: you have to be able to demonstrate in court that you explained to the customer prior to the purchase what the maximum stresses were that the structure could withstand and what its expected lifetime would be given proper maintenance, and you then have to demonstrate that when it failed it was either because (a) the maximum stresses had been exceeded, (b) the intended lifetime had been exceeded, or (c) it had not been properly maintained.

That's the way a free society works. In a free society, the economy is not "regulated." That means no path has been cleared through the economic system for "businessmen" who intend to "succeed" by means of fraud. Result: in a free society, businessmen of that sort cannot compete. They are either sent to jail, or else they are sued into impoverishment, and spend their lives working at their actual level of competence (e.g., cleaning other people's toilets), as they deserve.

That is the distinction which explains why the Big Bad Wolf, better known as Hurricane Andrew, turned the "modern" homes in south Florida into a sea of destruction, while leaving hundred year old homes mostly untouched.

Regarding compression perpendicular to the grain, you expressed doubt that it was a failure mode that had safety-related implications, and asked for an example.

First, we need to focus on some basic logic. To that end, imagine that you have a couple of white oak 2x4s, square cut on the ends, roughly 2 feet long. If you pick up one of them in each hand, hold one in a vertical orientation and the other horizontal, and rest the end of the horizontal member on the top of the vertical one with the 3.5 inch sides of both boards facing you, then the area of contact between them is going to be (1.5)(3.5) = 5.25 square inches. In compression perpendicular to the grain the elastic limit of clear wood, white oak in this case, would be 1073 psi, so speaking in terms of pure physics, if these two 2x4s are made of clear wood, with one functioning as a beam and the other as a column, the beam can transfer up to (5.25)(1073) = 5,633 lbs to the column without exceeding the beam's elastic limit.

Note, however, that this is true only because the angle between them is 90 degrees. Moreover, the angle between them will remain 90 degrees only so long as the only force involved is the downward acting force of gravity. As soon as a lateral force enters the equation, the angle at the joint where the beam interfaces the column is going to change, even if the lateral force is very small and the change is very slight. Suppose, for example, that you hold the vertical board with your left hand so that it stands on your kitchen table and push the horizontal member to the left with your right hand, allowing the vertical member to lean to the left. Result: as it leans, an angle will open up between the two boards and the area of contact between them will drop from 5.25 square inches to a very small area, a fraction of a square inch. The "beam," at that point, will be resting on the edge of the "column," and the area of contact between them will be close to zero.

Something similar happens throughout every wooden structure, to every joint between a wooden beam and the column that supports it, when the wind blows: as the structure leans in the downwind direction, however slightly, the areas of contact between the beams and columns try to decrease. However, since they are bearing heavy gravitational loads, what actually happens is that pressure decreases on the lee side of the surface of contact and increases on the windward side. It is only when the lateral force exerted by the wind is extreme that an actual angle will open up between the beam and the column on the lee side of the interface. The effect, however, is the same as if an angle had opened up, because the pressure across the windward side of the interface rises above the value it had when the wind was not blowing, toward the elastic limit of clear wood. Somewhere along the way, if the lateral force keeps rising, each individual beam in the structure will first reach, and then exceed, its own individual elastic limit. Beams the ends of which are flawed, containing knots, pitch pockets, separated grain, etc., will reach their individual elastic limits first, in an order determined by their positions within the structure and the extent to which they, individually, are flawed. But finally even those beams which have ends consisting of clear wood and which are favorably positioned within the structure will reach their elastic limits. At some point in the process, joints between beams and columns are going to begin to give way, and very shortly afterwards the structure is going to come apart. And the failures, in almost every case, are going to be due to the ends of the beams giving way, rather than to failure of the columns. The reason: Newton's third law says that contact forces are always equal and opposite. That means if an upward pressure of 1073 psi is exerted on a beam made of white oak, an equal and opposite downward pressure is exerted on the column that supports it. And since wood is vastly stronger in compression parallel to the grain than it is in compression perpendicular to the grain, it is virtually always going to be the ends of the beams that will give way under heavy lateral loading, thereby allowing the joints between beams and columns to tear apart, and initiating the failure of the structure as a whole. Moreover, none of the widely-used beam-column connectors address this issue in the slightest, and the unconventional ones that do address it use metal corner braces to freeze the joints into a right angle--which means: when lateral loads rise, both the ends of the beams and the tops of the columns will experience rising stresses acting perpendicular to the grain. Result: the problem is not solved; it is merely shifted to a different location.

Bottom line: the only way to address this problem is the way it was addressed a hundred years ago: you need much larger safety factors for compression perpendicular to the grain, and you need to accept the larger columns and the associated rise in costs which such safety factors imply. That's why I said several posts back that the implicit safety factors hidden in the design-value tables for compression perpendicular to the grain are too low--so low, in fact, as to be ridiculous.

That's why Hurricane Andrew disproportionately destroyed the new homes and spared the very old ones.

That's why America was a far safer place when it was a free country with an unregulated economy, operating under the principle of laissez faire, than it is today, with an economy in which "regulation in the public interest" has been used to clear a path, in virtually every area of economic activity, for human predators whose "business plan" is to "succeed" by means of fraud.

 

[nutte]

I was going to read that, but I think I can read the whole NDS and all the years of research on compression perpendicular to the grain in less time.

 

[frv]

Wow, Hickory.

You certainly are retired, aren't you.

I'm sorry you wasted your time, but I'm just not that interested in what you have to say.

Brevity, at times, can be your friend.

 

[OHIOMatt]

In reference to the post by Cadair. I am not sure if it was a coincidnece or you inteded to select #2 SYP in your post. Anyway, it is this grade and size of lumber that has ignited the controversy surrounding the design values of syp. The latest information that I have seen indicates that all visually graded syp is going to see all design properties reduced by at least 25%.

Appartently over the past 17 years, the selected, tested samples have shown a steady decline properties. While I do not belive that the properties were determined in a smoke filled back room, I would like to know how on one day, the design values can change by 30%. A floor I designed last month, today would show an over stress of 30% and deflection well out of tolerence.

A month ago I would have said that the way we are doing things today are better than the way we did them years ago. Now, I am not so sure.

 

[Cadair]

I intentionally picked that. Even the "experts" really don't know what is going on and are beating on the data. My point was that if people who's profession is to do this aren't always sure, what is the chance of an engineer that has a lot more on his plate getting it right?

Hopefully we will know about SYP come Jan. 5th when ALSC rules. I could see it going either way.

 

[LonnieP]

OHIOMatt and Cadair,

Since the late 50's I've watched the allowables for sawn lumber reduced. The 'In grade testing' program gave an explaination for some of the reductions. I suspect better testing methods was another. A timber design speaker said values would continue to fall as use of farm grown lumber became more common. Something to do with spacing of the growth rings and how it effects the Modulus of Elasticity, or something like that. We are definately living in 'interesting times'.

 

[Cadair]

LonnnieP,

I would expect that to be the case, but it doesn't explain the sudden, drastic change. Nor does it explain why only the visual grades are effected.

I would expect the values to steadily change with time and even jolt with new test methods. But this is a jolt without a new test method. Them saying it is a change in source material doesn't make sense.

 

[msquared48]

At the risk of being eaten alive here, I do not think that perpendicular to grain is much of a safety issue, most of the time. I see it as more of an Architectural finish issue, primarily affecting sheetrock and overall appearance.

Like soils, wood will compress until the bearing stress needed is generated, up to a point of course. This will result in settlement and unevenness in the structural system. It can, in certain circumstances, result in a reduced section, reducing the bending and shear strength. The settlement may also induce leaks in plumbing or roof structures, further affecting the structure negatively. However, if all you are doing is bearing on a foundation sole plate, I see no serious life threatening structural issue.

Mike McCann
MMC Engineering